The present invention relates to an integrated process for the synthesis of urea and ammonia.
The integrated urea-ammonia processes in which CO.sub.2 contained in the gaseous feed sent to the NH.sub.3 synthesis is absorbed by aqueous ammoniacal solutions, can be described in the following manner.
The feed gas utilized in the synthesis of NH.sub.3 contains N.sub.2, H.sub.2, CO.sub.2 and small amounts of Ar, CH.sub.4 and CO and is compressed to the ammonia synthesis pressure and fed to a primary absorber, wherein a selective absorption is carried out by means of an ammoniacal solutions obtained in various ways. In any case, the ammonia stream leaving the synthesis is utilized in order to remove CO.sub.2 and to form ammonium carbamate.
The ammoniacal solution is fed to the absorber bottom and to the absorber top for increasing as much as possible the CO.sub.2 absorption and for limiting, as much as possible, the evaporation of NH.sub.3 due to heat developed during the carbamate formation.
Ammonium carbamate is subsequently sent to a urea synthesis zone wherein it is dehydrated, transforming partially into urea.
The urea-ammonium carbamate mixture which leaves the urea synthesis zone is fed to a stripping zone wherein by the action of heat the carbamate is transformed into CO.sub.2 and NH.sub.3 and by means of a stripping agent said compounds are recovered and sent back to the synthesis zone after, or without, previous condensation.
An aqueous solution of urea containing a small amount of carbamate is discharged from the stripping zone.
The pressure of the stripping zone for the urea solution is the same as that utilized in the formation of the carbamate and the synthesis of the urea. The pressure under which the urea solution is maintained is gradually decreased, generally in two stages, down to atmospheric pressure. At the first stage of lowered pressure (in general at about 16 atmospheres) the urea solution is distilled, obtaining as overhead products, water, ammonia and CO.sub.2 which, after condensation-rectification in a single column, are separated into a gaseous phase constituted by NH.sub.3 and a liquid phase constituted by a concentrated solution of ammonium carbonate which is sent back to the urea synthesis zone. The urea solution leaving the first distillation stage at the lowered pressure is fed to a second distillation stage at a further reduced pressure (in general at about 4 atmospheres) which separates as overhead products, ammonia, water and CO.sub.2 which after condensation give a weakly concentrated ammoniacal solution of ammonium carbonate. This last solution in the continuation of the description will be referred to as "ammoniacal solution" is recycled to the condensation-rectification column in order to recover liquid ammonia and carbonate. The carbonate is recycled together with the previously mentioned carbonate to the urea synthesis zone.
From the chemical-physical data known from the literature and confirmed by the experimental values obtained with apparatus analogous to that are used for industrial applications it is known that the residual amount of CO.sub.2 in the gas leaving the absorber depends upon, in addition to the total pressure of the system, the composition of the obtained carbamate solution, the amount of NH.sub.3 in excess of the stoichiometric value necessary to form carbamate present in the gaseous and liquid phases, the concentration of the absorbing solution, and the temperature of the liquid and gaseous phases.
Also, by utilizing as the CO.sub.2 absorbing solution, the ammoniacal solution obtained by absorption of the NH.sub.3 from the synthesis, or also the aqueous urea and the NH.sub.3 solutions, the amount of residual CO.sub.2 contained in the gas is still high. Because of this, the recovery of a portion, or all of the NH.sub.3 contained therein by condensation by cooling is not possible. Because of this, the NH.sub.3 remains in the gas, and before being eliminated by absorption, is fed to a subsequent stage of elimination of CO.
Said absorption can be carried out also in the previously described NH.sub.3 absorption stage.
A drawback is presented in absorbing the evaporated ammonia because in order to do this an additional amount of water over and above that necessary to absorb the ammonia coming from the primary synthesis is required.
The final result is that the amount of water which is utilized at first in the absorption of NH.sub.3, then in the absorption of CO.sub.2, and at last together with carbamate in the synthesis of urea is particularly high. Because of this, the urea synthesis reaction, stripping and recovery of CO.sub.2 and NH.sub.3 not converted into urea, in the plant sections downstream of the stripper are remarkably influenced in a negative sense. There is in fact a lowering of the conversion of carbamate to urea, an increase of hydrolisis of urea in the stripping stage and in the subsequent distillation stages of the urea solution, an increase of steam consumed, of the amount of cooling water and energy required, and an increase of the cost of the apparatus constituting the plant, and so on.